fix #27040, promote the upper bound of a type parameter

pkg/analyzer/lib/src/generated/resolver.dart

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index e831fc09e62b69a94cf9608dae4952514bc37ec4..5bedcf95d4448eb99f810bfffff952dab19d629a 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -6887,9 +6887,10 @@ class ResolverVisitor extends ScopedVisitor {
       potentialType ??= DynamicTypeImpl.instance;
 
       // Check if we can promote to potentialType from type.
-      if (typeSystem.canPromoteToType(potentialType, type)) {
+      DartType promoteType = typeSystem.tryPromoteToType(potentialType, type);
+      if (promoteType != null) {
         // Do promote type of variable.
-        _promoteManager.setType(element, potentialType);
+        _promoteManager.setType(element, promoteType);
       }
     }
   }